1. Field of the Invention
The present invention relates to a touch sensor for a probe, which, for example, is used when measuring micro-configuration of the surface of a test piece by equipment such as a micro-configuration measuring device and a surface roughness measuring device or inner configuration of a hole by a small hole measuring device.
2. Description of the Related Art
Conventionally, micro-configuration measuring devices are used when examining a test piece for research and development purposes as well as for production activities in the fields of precision machining or semiconductor manufacturing. The device measures micro-dimensions such as a surface roughness or a step on the machined surface and a thickness of a thin film by a vertically oscillating a stylus, which is brought in contact with and moved about the test piece. The change in the vertical oscillation of the stylus is then converted into an electrical signal to be read.
One example of such a mechanism involving a stylus used in micro-configuration measuring devices as described above is a touch sensor, which is disclosed in Japanese Patent Laid Open No. 2001-91206.
In FIG. 6, this touch sensor 10 includes a stylus holder 11, a stylus 12 which is held by arms 13, 14, 15 and 16 and has a tip 12A making contact with the test piece, and a couple of piezoelectric elements 19, one of which is attached to the stylus on one side and the other of which on the side opposite thereto. Each piezoelectric element 19 is made of two parts, one being an oscillation means 17 and the other a detection means 18, joining at the center.
Given such a structure, if an electrically alternating signal of an appropriate oscillation frequency is applied to the oscillation means 17, then the stylus 12 starts oscillating in a resonating manner in the axial direction. If, in this resonating state, the tip 12A of stylus 12 makes contact with the test piece, then the resonating state changes, and this change of state can be detected by monitoring output from the detection means 18.
In precision measurement where micro-configuration is measured by using a touch sensor described above, it is important that measuring force acting between a test piece and the tip of a stylus be controlled below a prescribed value; the test piece and the tip not be damaged; and movement of the stylus tip accurately reflect the surface configuration of the test piece. Accordingly, a probe, which is equipped with a mechanism of controlling the measuring force below a prescribed value, is available.
One example is a probe for a micro-configuration measuring device disclosed in the U.S. patent application Ser. No. 09/805309.
In FIG. 7, this probe for micro-configuration measuring device is made of the above-mentioned touch sensor 10 and a fine motion mechanism 21 using a piezoelectric element (PZT), which are coupled together along the axis of oscillation of the stylus 12 and, as a whole, are attached to a movable support member 22.
Given such a structure, if an electrically alternating signal, which is characterized by the oscillation frequency and the oscillation voltage, are sent from the oscillator 3 to the oscillation means 17, then the stylus 12 starts oscillating in a resonating manner along its axis. If, in this state, the stylus tip 12A makes contact with the test piece W, the resonating state of the stylus 12 changes. Accordingly, by monitoring output from the detection means 18 indicating this change, the contact between the stylus tip 12A and the test piece W can be detected. Output from the detection means 18, which is designated as the detection signal DS1, is sent out to a detecting circuit 4. The detecting circuit 4 converts the detection signal DS1 into the detection signal DS2. The detection signal DS2 is filtered by a filter 51 to remove noises and sent out to a signal processing unit 62 as the detection signal DS3. The signal processing unit 62 computes a difference between the detection signal DS3 and a threshold which determines the measuring force and sends out the result to a controller 61. The controller 61 drives the fine motion mechanism 21 via a PZT driver 72 based on the result received. This system of controlling fine movement described so far allows the detection signal DS3 to be maintained constant with respect to any bumps and dips on the test piece W when the fine motion mechanism 21 and the test piece W are in relative motion for scanning.
In order to be successful in making non-destructive measurement on a test piece such as a silicon wafer, it is important how much the measuring force can be minimized. And, in order to minimize the measuring force, it is necessary that sensitivity of a touch sensor be boosted or the threshold be raised. What was conventionally attempted for the minimization of the measuring force is the boosting of the sensitivity of touch sensors through modification of their structure. However, such modification was not able to produce satisfactory results regarding the performance of micro-configuration measuring devices.
A principal purpose of the present invention is to provide a touch sensor, which is capable of minimizing measuring force and making non-destructive measurement without damaging micro-configuration of the surface of a test piece.
Here, a relationship between the measuring force and the detection signal DS3 in FIG. 7 will be described.
FIG. 1 illustrates a setup for an experiment on static characteristics of the detection signal DS3 during contact and non-contact conditions. A constant oscillation frequency is sent out from the oscillator 3 so that the stylus 12 oscillates in the direction of its axis. Then, a drive voltage from the PZT driver 7 is made to gradually increase so that the stylus 12 is brought closer to the test piece W. As the stylus 12 is further brought closer to the test piece W so that the tip 12A of the stylus 12 starts making contact, force inflicted onto the tip 12A gradually increases, and, at the same time, oscillation amplitude of the stylus tip 12A gradually decreases.
FIG. 2 illustrates a relationship between the force inflicted on the stylus tip 12A (the measuring force) and the detection signal DS3. The horizontal axis and the vertical axis of the graph represent the force inflicted on the stylus tip 12A and the detection signal DS3, respectively. The detection signal DS3 is maximum in the non-contact region. Assuming that where the detection signal DS3 begins decreasing is the point of contact, the force inflicted on the stylus tip 12A increases and the detection signal DS3 decreases, both starting at the point of contact. A slope in this region is called a sensitivity gradient. The higher the sensitivity of the touch sensor is, the steeper the slope of the graph is. From the graph, it can be seen that, since the force on the stylus (the measuring force) is determined by the sensitivity gradient and the threshold, the minimization of the measuring force can be achieved either by boosting the sensitivity gradient or raising the threshold.
In conventional probes for micro-configuration measuring devices, the minimization of the measuring force was attempted by boosting the sensitivity of the touch sensor through modification of its structure. For example, in the touch sensor illustrated in FIG. 6, by designing flexural rigidity of the arms 13, 14, 15 and 16 to be lower than that of the stylus 12 in the axial direction, or by arranging the arms 13, 14, 15 and 16 symmetrically about the axis of the stylus 12, flexural vibration of the stylus 12 with respect to its axis was prevented, thereby increasing the sensitivity of the touch sensor 10. Since the sensitivity gradient becomes steeper for such design or arrangement, the force on the stylus tip 12A (the measuring force) can be minimized, accordingly.
An inventor of the present invention has conceived an idea that by adjusting the oscillator which sends out an electrically alternating signal to an oscillation electrode or by adjusting the detecting circuit which detects changes of oscillation as the stylus tip touches the test piece, the above-mentioned purpose can be achieved, instead of modifying the structure of the touch sensor so that it has a steep sensitivity gradient. Specific structure thereof is as follows.
In the present invention, a touch sensor includes a stylus tip which makes contact with a test piece; a piezoelectric element for oscillating the stylus tip; an oscillator for oscillating the piezoelectric element; and a detecting circuit for detecting a change of a quantity of state which occurs when the stylus tip and the test piece make contact. This touch sensor is characterized in that, by adjusting an electrically alternating signal from the oscillator, a measuring force produced between the stylus tip and the test piece upon contact can be adjusted.
According to the present invention, the strength of the measuring force is adjusted by adjusting the electrically alternating signal from the oscillator. For example, if the oscillation voltage is reduced, then the sensitivity gradient becomes steeper, thereby adjusting the measuring force. Incidentally, it is clear from the result (FIG. 4) to be described later that by reducing the oscillation voltage, the sensitivity gradient can be made steeper. For example, assuming that the measuring force is constant, a degree of attenuation for the detection signal is greater, i.e., the sensitivity gradient is steeper when the oscillation voltage is small and the force for oscillation is small. Therefore, there is no need to modify the structure of the touch sensor for steeper sensitivity gradient and greater sensitivity as was done conventionally. This promises the possibility of breaking through the limit of minimization of the measuring force through the improvement of sensitivity of the touch sensor.
Moreover, since the adjustment of the measuring force is done by adjusting the electrically alternating signal from the oscillator, the present invention can be implemented by using a touch sensor equipped with a conventional stylus, thereby greatly improving the general versatility.
In the present invention, the adjustment of the electrically alternating signal is preferably accomplished by adjusting at least one from the group including the oscillation voltage, the oscillation frequency and the oscillation phase.
The adjustment of the measuring force can be done by adjusting the electrically alternating signal, specifically, by adjusting the oscillation voltage from the oscillator, by displacing the oscillation frequency from the resonance frequency of the stylus, or by adjusting the oscillation phase.
In the present invention, the measuring force can preferably be adjusted by monitoring any one from the group including the electrically alternating signal, the quantity of state in the detecting circuit, and an oscillating amplitude of the stylus tip.
Given such a structure, if a relationship among the measuring force, the electrically alternating signal and the threshold is known in advance, then monitoring any one from the group including the electrically alternating signal, the quantity of state in the detecting circuit and the oscillation amplitude of the stylus tip allows the measuring force to be adjusted both closely and precisely.
In the present invention, a touch sensor preferably further includes a low-pass filter in the detecting circuit, and the measuring force can be adjusted by adjusting the electrically alternating signal and the time constant of the low-pass filter based on a prescribed relationship.
Given such a structure where the low-pass filter has been added, noise added to the quantity of state in the detecting circuit during contact and non-contact conditions can be reduced. Accordingly, the threshold can be raised above a prescribed value and the measuring force can be adjusted, i.e., reduced. Moreover, by adjusting the electrically alternating signal and the time constant of the low-pass filter based on a prescribed relationship, an increase of noise due to the adjustment of the electrically alternating signal can be prevented through the adjustment of the time constant of the low-pass filter. Accordingly, the measuring force can be adjusted with the threshold being raised above a prescribed value, thereby reducing the measuring force.
In the present invention, the above-mentioned prescribed relationship is preferably an inverse proportionality between the oscillation voltage of the electrically alternating signal and the time constant of the low-pass filter.
Given such a structure, since the oscillation voltage of the electrically alternating signal is inversely proportional to the time constant of the low-pass filter, fluctuations in the noise can easily be predicted from the oscillation voltage of the electrically alternating signal. Furthermore, by determining the time constant of the low-pass filter, the measuring force can efficiently be adjusted.
In the present invention, the adjustment of the time constant of the low-pass filter is preferably automatically performed based on the above-mentioned prescribed relationship.
Given such a structure, since the time constant of the low-pass filter is automatically adjusted based on the above-mentioned prescribed relationship, by utilizing such prescribed relationship, the time constant of the low-pass filter can automatically be determined from the electrically alternating signal, thereby adjusting the measuring force.